1. Field of the Invention
The invention relates to high pressure discharge lamps, and more specifically, to high pressure discharge lamps having thick film resistors included within the outer lamp envelope. The invention also relates to an improved thick film resistor for operation in a vacuum, and especially in a high temperature vacuum, environment.
2. Description of the Prior Art
High pressure discharge lamps, including high pressure sodium vapor discharge (HPS) lamps, metal halide lamps, and mercury vapor lamps, often have multiple power-dissipating resistors included in the lamp circuit and within the outer lamp envelope. Power resistors are typically considered to be resistors which dissipate greater than about 1 watt during operation.
One or more resistors often form part of a starting circuit within the envelope for starting the discharge vessel, or arc tube. In metal halide lamps and mercury vapor lamps, the starting circuit typically includes an auxiliary electrode in the discharge vessel adjacent one of the discharge electrodes, which auxiliary electrode is electrically connected to the opposite discharge electrode through a current limiting resistor. Often a bimetal switch is in series with the current limiting resistor to remove the resistor and the auxiliary electrode after starting and stabilization of the discharge arc. A common starting circuit for HPS lamps includes a glow starter switch in series with a current limiting resistor and a bimetal, all of which are in parallel circuit with the arc tube. Resistors used in this type of HPS starting circuit typically have a resistance of over a hundred ohms and dissipate high power, on the order of several hundred watts. They are electrically disconnected from the arc tube circuit by the glow switch shortly after ignition of the discharge arc, typically within approximately 20 seconds after initial application of an electric current to the lamp. Several minutes after ignition of the discharge arc, the bimetal opens in response to heat from the discharge vessel to physically and electrically disconnect the glow switch and starting resistor from the arc tube circuit.
Additional resistors may be in the arc tube circuit during lamp operation to improve lamp performance. For example, U.S. Pat. No. 4,258,288 (Michael et al) discloses a metal halide high-intensity discharge (HID) lamp for connection to a constant wattage (CWA) ballast of the type having a transformer with a secondary winding in series with a capacitor. The lamp has an internal voltage-doubler starting circuit with two resistors in series with a bimetal switch. The bimetal switch disconnects the starting circuit and auxiliary electrode after starting of the lamp. The lamp also has a third power resistor in series with the arc tube which reduces the phase shift between the lamp voltage and the ballast open circuit voltage during lamp warm-up. The resistor increases the maximum sustaining voltage to the lamp when the lamp current is zero, thereby preventing flicker and extinguishment of the arc.
Japanese Kokai 1-211896 shows an unsaturated high pressure sodium discharge lamp suitable for operation on a CWA ballast. The lamp has a resistor in series with the arc tube to reduce the reignition voltage of the arc tube to prevent flicker of the arc, which otherwise occurs under certain operating conditions of the ballast and lamp. Because the resistor is in series with the arc tube, it operates continuously during lamp operation after ignition of the discharge arc and dissipates considerable power, approximately 15 watts for a 150 watt lamp.
Various types of power resistors have been used in high pressure discharge lamps, including filament resistors and miniature incandescent lamps. Filament resistors used in starting circuits have the disadvantage that they generally must be long, and as a result are formed into coils and/or suspended in zig-zag form, causing space and mounting problems within the lamp envelope. They are also sensitive to vibrations and mechanical shock, and consequently are a source of lamp failures. The use of miniature incandescent resistor lamps has typically been confined to continuous duty applications, such as the series flicker elimination resistor in unsaturated HPS lamps. Although the life of miniature lamps for this application must be longer than the life of the arc tube, typically greater than 24,000 hours, their filament is also subject to failure from shock and vibration and may be the cause of lamp failure.
Recently, ceramic thick film resistors, wherein a thick film resistive element such as tungsten is disposed on a ceramic substrate, have been used in starting circuits for HID lamps. For example, U.S. application Ser. No. 07/378,879 filed Jul. 12, 1989 shows a thick film resistor in a starting circuit for high pressure sodium discharge lamps. J.P. Kokai 56-73856 discloses a thick film resistor as a starting resistor for metal halide lamps and high pressure sodium discharge lamps. Thick film resistors are suitable for starting circuits because they reliably dissipate the required several hundred watts for the period just prior to lamp starting (.congruent.20 sec) while having a long life.
However, the use of thick film resistors in HID lamps for continuous duty operation has not been evident. For example, J.P. Kokai 1-211896 shows a tungsten filament resistor for flicker elimination. The resistor is mounted to the lamp frame at the dome end of the outer envelope, which requires a complicated construction and causes shadowing of the light emitted from the lamp. Even where a thick film resistor has been employed in a starting circuit for very high power dissipation prior to starting, separate resistors have been used for lower power applications. For example, J.P. 56-73856 shows a conventional carbon resistor in series with the auxiliary electrode in addition to the thick film short-duty starting resistor.
Accordingly, prior HID lamps having multiple resistor means have employed separate resistor components for each resistor means and suffer from the complexity, cost, and reliability problems associated with handling, mounting, and connecting multiple resistor components to other elements within the lamp envelope. Additionally, although not discussed in J.P. Kokai 1-211896, CWA mercury ballasts do not have a starter. In unsaturated HPS lamps for operation on this type of ballast, it is desirable to include a starting circuit within the lamp envelope. However, mounting of the starting resistor, glow starter, and bimetal switch near the base end of the lamp envelope is space consuming and typically requires multiple welds to the lamp frame. Mounting of an additional flicker elimination resistor component on the lamp frame between the discharge vessel and the lamp stem has not been practicable. For mercury-retrofit HPS lamps, the light center length of the arc tube measured from the base should equal the light center length of the mercury vapor lamp which it replaces to obtain optimum optical performance in the luminaire. Thus, it is not feasible to position the arc tube further from the base to obtain more mounting space on the frame in such a lamp.